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Power line communication apparatus and method for operating the samePower line communication apparatus and method for operating the same description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20080088418, Power line communication apparatus and method for operating the same. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims priority from Japanese Patent Application No. JP 2006-227712 filed in the Japanese Patent Office on Aug. 24, 2006, the entire content of which is incorporated herein by reference. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates to a power line communication apparatus to which a band window for band reservation and a communication band for data communication are alternately assigned at predetermined intervals, as well as to a method for operating that power line communication apparatus. [0004] 2. Description of the Related Art [0005] There are a number of types of communication apparatuses that utilize the power line for communication purposes. Such communication apparatuses may illustratively adopt, as their modulation system, ASK (amplitude shift keying), FSK (frequency shift keying), or PSK (phase shift keying). As their transmission system, the apparatuses may typically select the single carrier scheme, OFDM (orthogonal frequency division multiplexing), or spread spectrum communication system. [0006] The communication apparatuses of such different systems are known to be incapable of communicating with one another. For example, operating on the same OFDM principle, the apparatuses are unable to communicate with one other if they differ in terms of any one of such system specifications as occupied bandwidth, subcarrier cycles, subcarrier modulation, synchronizing signal, encoding method, access control, and data format. In the description that follows, the term "different systems" will refer to systems with differences therebetween in terms of their communication specifications. [0007] If apparatuses of different systems incommunicable with each other are installed in the same home, their outgoing signals can collide with each other, making communications difficult. In the worst case, all apparatuses could become incapable of communicating with one another. As power line communication apparatuses come into general use from now on, it will become important to devise means and arrangements to minimize or forestall interference between the communication apparatuses of different systems so that they can coexist in a delimited environment. [0008] In order to ensure the coexistence of different wireless communication systems, there have been proposed techniques for controlling the timings of communication between such systems using a signal (beacon signal) that can be exchanged therebetween. One such technique is disclosed illustratively in Japanese Patent Laid-open No. 2003-258812, referred as Patent Document 1. [0009] In a common wireless LAN setup, the beacon signal is typically used as follows: each master station periodically sends out a wireless beacon signal (a kind of OFDM-modulated control packet). A slave station receives the beacon signal on all wireless channels, and selects an optimal master station based on the received beacon signal. The slave station proceeds to request authentication from the selected master station. In turn, the master station determines whether or not to authenticate the slave station requesting connection to the network of its own. If the requesting slave station is successfully authenticated, the master station exchanges management signals with the authenticated station and thereafter treats the slave station as a legitimate wireless LAN terminal. [0010] That is, the beacon signal is a sort of intra-system control signal generated by the master station as a wireless identification signal that identifies the network managed by the master station in question and the devices connected to that network (the network and the connected devices will be generically referred to as the system hereunder). The invention disclosed by Patent Document 1 involves furnishing the beacon signal, commonly defined between systems, with detailed control information such as transmission parameters so as to secure the coexistence of different systems. [0011] In the field of power line communication, there have been proposals to define a CDCF (commonly distributed coordination function) signal as a coexistence control signal (simply called the coexistence signal) recognizable by modems (e.g., PLC modems) or other suitable communication apparatuses of different systems to ensure their coexistence, the CDCF signal being used as the basis for working out the system specifications for band reservation. One such proposal is found in Non-Patent Document 1 titled "Overview of CEPCA and Approach to PLC Standardization," Jul. 19, 2006 (presented by Mark Eyer, Sony), pp. 26-28. [0012] In power line communication, the use of existing power lines has made system standardization a complex and difficult exercise. It is preferred that the specifications for system coexistence be as simple as possible. Although the CDCF signal is based on the OFDM system on the assumption that it has been adopted by a majority of today's high-speed modems, the modems themselves cannot exchange detailed information. The most they can do is to notify one another of the presence of a master station that performs band reservation. SUMMARY OF THE INVENTION [0013] Patent Document 1 describes how the beacon signal used commonly by different systems allows each of the systems to detect the presence of a wireless network and to verify its ongoing utilization status. In the field of power line communication, the coexistence signal (e.g., CDCF signal) has similar functions but, furnished with detailed information, may not be accepted by many apparatuses because not only new but also existing facilities have been used to set up PLC communication networks. This can be an impediment to promoting the power line communication scheme. In the power line communication specifications, the coexistence signal may be defined as a signal that should be significant in two aspects: the very presence or absence of the signal, and the timing of the signal in a dedicated band (band window). [0014] Non-Patent Document 1 discloses the significance of the regions of times or frequencies assigned within band windows. However, there is neither description of the initial settings for power-up in coexistence mode or standalone mode, nor explanation of the conditions for transition from one mode to the other following the activation. [0015] With a view to preventing interference between different systems, coexistence mode may be selected upon receipt of the coexistence signal even if there is no possibility of such interference. Coexistence mode involves giving priority to averting the collision of data by placing constraints on the times or frequency bands for use in data transmission. In that sense, establishing coexistence mode in the absence of the possibility of data collision can unnecessarily reduce the capacity of data transmission, which can be a major disadvantage. [0016] The present invention has been made in view of the above circumstances and provides a power line communication apparatus which may be capable of establishing a suitable mode at power-on and effecting transition from one mode to another so as to prevent data collision (i.e., interference between systems) while ensuring a sufficient capacity of data transmission, as well as a method for operating that power line communication apparatus. [0017] In carrying out the present invention and according to one embodiment thereof, there is provided a power line communication apparatus which may be assigned a band window and a communication band in alternate and cyclical fashion, the band window being one to which a coexistence signal for band reservation is assignable, the communication band being one to which desired information is assignable, the power line communication apparatus may include a reception processor configured to receive and process a signal from a power line communication network; a coexistence signal detector configured to detect, from the signal received by the reception processor, the presence and signal strength of the coexistence signal, and the timing of the band window which is assigned the coexistence signal; a coexistence signal generator configured to generate a coexistence signal; a transmission processor configured to process a signal and transmit the processed signal onto the power line communication network; and a controller configured to effect control to establish either coexistence mode or standalone mode at power-on, the coexistence mode being one in which the band reservation is made by causing the coexistence signal generator to generate the coexistence signal and by having the transmission processor transmitting the generated coexistence signal, the standalone mode being one in which the band reservation utilizing the coexistence signal is not carried out; the controller being further configured to effect control to make transition from one of the two modes to the other; the establishment of one of the two modes and the transition therebetween being controlled on the basis of the coexistence signal being detected by the coexistence signal detector, on the basis of the signal strength of the coexistence signal if the presence of the signal is detected, or on the basis of both the signal strength of the coexistence signal and the timing of the band window which is assigned the coexistence signal. [0018] Preferably, the controller may establish the standalone mode if the coexistence signal is not detected, or if every coexistence signal detected has a signal strength of less than a predetermined level P3; and the controller may establish the coexistence mode if at least one coexistence signal is detected at power-on and if the signal strength of at least one of the detected coexistence signals is at least on the level P3. [0019] Preferably, if all coexistence signals detected at power-on are found to have signal strength levels less than the level P3, then the controller may control the coexistence signal generator and the transmission processor in such a manner as to transmit a coexistence signal at the timing of the band window of each detected coexistence signal. [0020] Preferably, if one band window is detected which is assigned a coexistence signal with a signal strength of at least the level P3 upon establishment of the coexistence mode, then the controller may control the coexistence signal generator and the transmission processor in such a manner as to transmit a coexistence signal through a band window at the same timing as that of the detected band window; and if a plurality of band windows are detected each of which is assigned a coexistence signal with a signal strength of at least the level P3 upon establishment of the coexistence mode, then the controller may control the coexistence signal generator and the transmission processor in such a manner as to standardize the timings of the detected plurality of band windows and to transmit a coexistence signal through a band window at the standardized timing. [0021] Preferably, with the standalone mode in effect, if the coexistence signal is not detected or if the signal strengths of all detected coexistence signals are less than a predetermined level P2, then the controller may maintain the standalone mode; with the standalone mode in effect, if a plurality of band windows are detected each of which is assigned a coexistence signal with a signal strength of at least the level P2, then the controller may control the coexistence signal generator and the transmission processor in such a manner as to standardize the timings of the detected plurality of band windows, transmit a coexistence signal through a band window at the standardized timing, and effect transition to the coexistence mode; and with the standalone mode in effect, if one band window is detected which is assigned a coexistence signal with a signal strength of at least the level P2, then the controller may effect transition to the coexistence mode without standardizing the timing of the detected band window. 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